In impact line printers, a plurality of print hammers are arranged in a row parallel with a continuous type carrier such as a flexible type belt or band with engraved characters. The carrier is supported by a pair of spaced drive rolls or pulleys and has a straight section opposite the row of hammers. A platen behind the straight section provides support in opposition to impact forces from the hammers. A control system selectively operates the hammers in synchronism with the movement of the type. The operating control depends on the flight times of the individual hammers. The flight times will differ among hammers. In addition, the flight times are subject to variation over a period of use.
It has been a common practice to measure the average flight time of each hammer at the time of manufacture and installation. The flight times are remeasured periodically. Such remeasurement heretofore involved interruption of printing, the removal of the print medium and the replacement of the type carrier with a test bar. It has been proposed to place transducer devices behind the carrier or within the platens. Examples of this are seen in U. S. Pat. No. 3,872,788 and the IBM Technical Disclosure Bulletin of January 1980, p. 3673, Vol. 22, No. 8B; of Mar. 1982, pp. 5084-5085, Vol. 24, No. 10; and of Nov. 1984, p. 3345, Vol. 27, No. 6. However, in such proposed arrangements, accuracy is print media limited and durability is too short.